Charge pump for a hydrostatic transmission

Abstract

A charge pump for a hydrostatic transmission, having a self-charge relief function, in which a first toothed wheel, a second toothed wheel, and a pump casing, housing therein the first and second toothed wheels, are molded of heat resistant resin material, thereby reducing the weight of the charge pump, stabilizing its charge relief function and reducing the wear of the lower surface of a center section. Check valves are housed in the pump casing so that operating oil sealed between a hydraulic pump and a hydraulic motor can always be maintained at a proper level, regardless of whether the charge pump is driven or not.

What is claimed is: 
     
       1. A hydrostatic transmission, comprising: a housing;   an oil sump formed in said housing;   a hydrostatic transmission disposed in said housing, including a hydraulic pump, a hydraulic motor, and a center section, on which said hydraulic pump and said hydraulic motor are disposed, wherein said center section includes a closed circuit for circulating operating oil between said hydraulic pump and said hydraulic motor and a supply port on a surface of said center section for supplying oil to said closed circuit;   a power input shaft supported by said housing, for driving said hydraulic pump;   a charge pump disposed in said oil sump driven by said power input shaft for sucking in oil from said oil sump and discharging said oil to said supply port of said center section;   a suction port for said charge pump fluidly connecting said charge pump to said oil sump;   an oil bore formed at a pump casing of said charge pump for opening a discharge port of said charge pump within said oil sump; and   a check valve disposed in said oil bore so as to allow said oil from said oil sump to flow only into said supply port of said center section through said discharge port of said charge pump.   
     
     
       2. A hydrostatic transmission according to claim 1, wherein said check valve is a single body and is molded of elastic material. 
     
     
       3. A hydrostatic transmission according to claim 2, wherein said check valve and said pump casing are molded of resin material. 
     
     
       4. A hydrostatic transmission according to claim 3, wherein said resin material of said check valve is softer than said resin material of said pump casing. 
     
     
       5. A hydrostatic transmission according to claim 1, wherein said oil bore includes an oil passage section, a valve seat disposed at one side of said oil passage section and a recess section disposed at another side of said oil passage, and said check valve comprises a poppet valve portion to correspond with said valve seat, a leg extending from a base of said poppet valve portion to be inserted through said oil passage section and a projection radially projecting from an utmost end of said leg to be engaged with said recess section, wherein said oil passage section is shorter than said leg. 
     
     
       6. A hydrostatic transmission according to claim 5, wherein said check valve is a single valve and is molded of elastic material so as to enable said leg to bend. 
     
     
       7. A hydrostatic transmission according to claim 6, wherein said check valve and said pump casing are molded of resin material. 
     
     
       8. A hydrostatic transmission according to claim 7, wherein said resin material of said check valve is softer than said resin material of said pump casing. 
     
     
       9. A hydrostatic transmission, comprising: a housing;   an oil sump formed in said housing;   a hydrostatic transmission disposed in said housing, including a hydraulic pump, a hydraulic motor, and a center section, on which said hydraulic pump and said hydraulic motor are disposed, wherein said center section includes a closed circuit for circulating operating oil between said hydraulic pump and said hydraulic motor and a supply port on a surface of said center section for supplying oil to said closed circuit;   a power input shaft supported by said housing, for driving said hydraulic pump;   a charge pump disposed in said oil sump driven by said power input shaft for sucking in oil from said oil sump and discharging said oil to said supply port of said center section;   an oil filter disposed in said oil sump;   a suction port for said charge pump open adjacent to said oil filter;   an oil bore formed at a pump casing of said charge pump for opening said discharge port of said charge pump within said oil filter; and   a check valve disposed in said oil bore so as to allow said oil from said oil filter to flow only into said supply port of said center section, wherein said check valve is a single body and is molded of elastic material.   
     
     
       10. A hydrostatic transmission according to claim 9, wherein said check valve and said pump casing are molded of resin material. 
     
     
       11. A hydrostatic transmission according to claim 10, wherein said resin material of said check valve is softer than said resin material of said pump casing. 
     
     
       12. A hydrostatic transmission, comprising: a housing;   an oil sump formed in said housing;   a hydrostatic transmission disposed in said housing, including a hydraulic pump, a hydraulic motor, and a center section, on which said hydraulic pump and said hydraulic motor are disposed, wherein said center section includes a closed circuit for circulating operating oil between said hydraulic pump and said hydraulic motor and a supply port on a surface of said center section for supplying oil to said closed circuit;   a power input shaft supported by said housing, for driving said hydraulic pump;   a charge pump disposed in said oil sump driven by said power input shaft for sucking in oil from said oil sump and discharging said oil to said supply port of said center section;   an oil filter disposed in said oil sump;   a suction port for said charge pump open adjacent to said oil filter;   an oil bore formed at a pump casing of said charge pump for opening said discharge port of said charge pump within said oil filter; and   a check valve disposed in said oil bore so as to allow said oil from said oil filter to flow only into said supply port of said center section;   wherein said oil bore includes an oil passage section, a valve seat disposed at one side of said oil passage section and a recess section disposed at another side of said oil passage, and said check valve comprises a poppet valve portion to correspond with said valve seat, a leg extending from a base of said poppet valve portion to be inserted through said oil passage section and a projection radially projecting from an utmost end of said leg to be engaged with said recess section, wherein said oil passage section is shorter than said leg.   
     
     
       13. A hydrostatic transmission according to claim 12, wherein said check valve is a single valve and is molded of elastic material so as to enable said leg to bend. 
     
     
       14. A hydrostatic transmission according to claim 13, wherein said check valve and said pump casing are molded of resin material. 
     
     
       15. A hydrostatic transmission according to claim 13, wherein said resin material of said check valve is softer than said resin material of said pump casing.